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Full text of "Handbook Of Chemical Engineering - I"

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PYROMETRY                                         469
chart. Power is obtained from a small motor with an adjustable governor attached which will maintain the speed quite uniform in spite of large variations in the line voltage.
The chart of this instrument is 10 in. wide and has rectangular coordinates. The scale is uniform if graduated in millivolts and departs from uniformity according to the temperature electromotive force relation of the couple, if calibrated in degrees of temperature. In multiple-point recorders a print wheel carrying four or more numbers is substituted for the pen. This wheel is turned in synchronism with the commutator so that the proper number is printed. A small inked roller turns in contact with the print wheel. The latter is depressed once a minute, a length of time sufficient to allow the balancing action to be completed after each couple is connected.
The standard galvanometer is sufficiently sensitive to work satisfactorily with a full scale range of 10 millivolts which gives a very open scale particularly for base-metal couples, where Ko in. of scale corresponds to 2KC. This recorder is also used with resistance thermometers in which case the electrical circuit takes the form of a Wheatstone bridge, and the scale can be opened until the entire range corresponds to so little as 2C. The zero of the scale can be adjusted to correspond to any fixed electromotive force so that the scale may be put within any range of temperature desired. The great adaptability of the instrument is readily apparent.
Transformation Point Indicators and Recorders.Such instruments are designed to indicate the temperature at which a chemical or physical transformation occurs. They are based on the fact that heat is absorbed or given of when such transformations take place. The simplest instrument will give a temperature-time curve of the sample of steel placed in a furnace and heated. The method is not applicable industrially and a modified form has been developed. Burgess has described1 the use of a neutral body and the differential couple, an arrangement first devised by Sir Roberts-Austen. The differential couple is mounted with one hot junction in the test piece and the other in a neutral body (one with no transformation points). The data obtained are curves of temperature of test piece vs. temperature difference between the test piece and the neutral body. These are mounted side by side in the furnace and heated and cooled as uniformly as possible.
The Brown transformation-point recorder is a roll-chart deflectional recorder in which the galvanometer moving clement has two coils on one winding. One of these coils is connected to the couple in the test piece and the other is connected to the differential couple. In operation the recorder and furnace are started and the test-piece couple is permanently connected to the galvanometer. The differential couple is connected through an interrupting contact such that the record shows two rows of dots, one of which gives the temperature and the other the temperature difference measured by the distance apart of the two curves. Base-metal couples and rather large test pieces are used. The instrument will show a very marked jog at the eutec-toid transformation point and by close observation the allotropic transformation point A3 may be detected in pure iron.
The Leeds & Northrup transformation-point indicator is a semiautomatic recorder giving a continuous curve of temperature vs. temperature difference. The complete apparatus includes the recorder, two wall-type double-suspension galvanometers, furnace, rheostat and thermocouples. The recorder is the potentiometer type with a drum chart. The potentiometer slide wire is mounted on the same shaft as the
i G. K. BUKGESS, On Methods of Obtaining Cooling Curves. Bureau of Standards Sci. Paper, No. 99.